1. Field of the Invention
The present invention relates generally to balancer shaft for use in multicylinder engines, and more particularly, to a balancer shaft provided with improved journals each turning in a bearing for use in a reciprocating piston engine of the multicylinder type.
2. Description of the Prior Art
In a reciprocating piston engine, a crank arm mounted rotatably on a crank shaft is provided with a balance weight for the purpose of suppressing engine vibrations which result from centrifugal force working on portions rotating eccentrically such as the crank arm and a crank pin or inertia force working on reciprocating portions such as a piston and a piston pin. The balance weight provided on the crank arm is generally contributive to cancel the engine vibrations. In a four-cycle, four-cylinder engine of the straight type, which is a fundamental form of the reciprocating piston engine, however, a secondary vibration of the engine at the frequency being twice as high as that of the revolution of the crank shaft, which results from inertia force working on reciprocating portions in the engine, is not sufficiently cancelled only by the balance weight provided on the crank arm.
With the view of suppressing such a secondary vibration of the engine, there has been proposed a balancing device employing a balancer shaft disposed to be rotatable in an engine, as disclosed in, for example, U.S. Pat. 4,028,963 which corresponds to the Japanese patent specification published after examination under publication No. 57-44863. The balancer shaft has a balance weight part and is arranged in parallel with a crank shaft to be driven thereby through a driving mechanism. When the balance weight part is provided on the balancer shaft in an appropriate form and balancer shaft is revolved at speed being twice as high as that of the crank shaft in the event of the four-cycle, four-cylinder engine of the straight type, the secondary vibration of the engine resulting from inertia force working on the reciprocating portions in the engine as mentioned above is cancelled.
In an engine equipped with such a balancing device, it is desirable that the balance weight part of the balancer shaft is disposed at a central area in the engine in the direction along the crank shaft and further journals of the balancer shaft are supported by bearings each provided at a portion of the engine that has high rigidity, namely, a bulkhead at a front or rear end or between cylinders in the engine, in order to avoid another vibration caused by the revolution of the balancer shaft with the balance weight part. Accordingly, in connection with the balance weight part and journals of the balancer shaft, such arrangements as shown in FIGS. 1 and 2 have been proposed as disclosed in also the aforementioned U.S. patent and further an arrangement using a balancer shaft having four journals, such as shown in the Japanese utility model specification published before examination under publication No. 56-62455, has been considered as shown in FIG. 3.
In the arrangement shown in FIG. 1, a balance weight part 2 of a balancer shaft 1 is disposed by the side of the second and third cylinders 3b and 3c which are positioned at the central area in an engine in the direction along a crank shaft, and the first and second journals 4a and 4b are provided on both end portions of the balance weight part 2 of the balancer shaft 1 to be supported by bearings provided in a bulkhead 5b between the first and second cylinders 3a and 3b and a bulkhead 5d between the third and fourth cylinders 3c and 3d, respectively. Further, an elongated shaft part 6 extends from the end portion of the balance weight part 2, at which the first journal 4a is provided, to engage with a driving mechanism (not shown in FIG. 1), and a third journal 4c is provided on the end portion of the elongated shaft part 6 to be supported by a bearing provided in a bulkhead 5a at the front end portion of a cylinder block.
In the arrangement shown in FIG. 2, a balancer shaft 1" has a balance weight part 2" corresponding to the balance weight part 2" of FIG. 1, which is provided with a first journal 4a" on the central portion thereof without any journal at both side portions thereof. The balancer shaft 1" is provided also with an elongated shaft part 6" corresponding to the elongated shaft part 6 of FIG. 1 and a second journal 4b" provided on the end portion of the elongated shaft part 6". The first and second journals 4a" and 4b" are supported by bearings provided in a bulkhead 5c between the second and third cylinders 3b and 3c and in a bulkhead 5a at the front end portion of a cylinder block, respectively.
Further, in the arrangement shown in FIG. 3, a balancer shaft 1' has a balance weight part 2' corresponding to the balance weight part 2 of FIG. 1, which is provided with first and second journals 4a' and 4b' at both side portions thereof, respectively, and an elongated shaft part 6' corresponding to the elongated shaft part 6 of FIG. 1, which is provided with a third journal 4c' at the end portion thereof. In addition, a fourth journal 4d' is provided on the central portion of the balance weight part 2'. The first to fourth journals 4a' to 4d' are supported by bearings in a bulkhead 5b between the first and second cylinders 3a and 3b, a bulkhead 5b between the third and fourth cylinders 3c and 3d, a bulkhead 5a in the front end portion of a cylinder block, and a bulkhead 5c between the second and third cylinders 3b and 3c, respectively.
However, there are the following problems in these arrangements shown in FIGS. 1 to 3. That is, in the arrangement shown in FIG. 1, since the balance weight part 2 has a relatively long span between the first and second journals 4a and 4b and therefore a relatively large bending moment resulting from centrifugal force works on the balance weight part 2, each of the first and second journals 4a and 4b or each of the bearings supporting the first and second journals 4a and 4b is apt to wear partially or seize. In the arrangement shown in FIG. 2, a bending moment working on the balance weight part 2' is reduced due to the first journal 4a" provided on the central portion of the balance weight part 2" to be supported by the bearing provided in the bulkhead 5c. However, since a half 2a" of the balance weight part 2" on the side opposite to the elongated shaft part 6" is substantially cantilevered and therefore swung with its revolutions, the first journal 4a" or the bearing supporting the first journal 4a" is apt to be aggravated to wear partially. Further, in the arrangement shown in FIG. 3, a bending moment working on the balance weight part 2' is reduced due to the fourth journal 4d' provided on the central portion of the balance weight part 2' to be supported by the bearing provided in the bulkhead 5c and the balance weight part 2' is prevented from swinging with its revolutions by the first and second journals 4a' and 4b' which are provided at both end portions of the balance weight part 2' to be supported by the bearings provided in the bulkheads 5b and 5d, respectively. However, the total amount of driving power loss resulting from resistance against sliding at each of the first to fourth journals 4a' to 4d' is increased in comparison with the other arrangements.
Incidentally, in general, each of a plurality of journals provided on a balancer shaft has the same diameter usually, and consequently it is not easy to cause the balancer shaft to engage with a cylinder block in such a manner that each of the journals on the balancer shaft is inserted correctly into corresponding one of bearings provided in a plurality of bulkheads of the cylinder block. Moreover, since the diameter of each of the journals is selected to be relatively large in order to permit the balance weight part to rotate eccentrically, the outer surface of each of the journals moves to slide on the inner surface of the corresponding bearing at relatively high speed and therefore each of the journals or the bearing engaging therewith tends furthermore to wear partially or seize.